The goal of this work is to improve our understanding of the biological function of vanadium, in general, and to investigate the reactivity of vanadium-containing proteins, more specifically. Vanadium is capable of existing in many oxidation states and the aqueous chemistry of vanadium is dominated by electron transfer, hydrolysis and/or polymerization reactions. Clearly the coordination environment of the vanadium will affect its reactivity. Towards our overall goal, we proposed to investigate the factors that govern the oxidation and reduction reactions of vanadium bound to macromolecules (eg., the naturally occurring V-bromoperoxidase and V substituted derivatives of transferrin, lactoferrin, ovotransferrin, carboxypeptidase A, carbonic anhydrase B, thermolysin, bleomycin, etc), and to examine the possibility that vanadium proteins catalyze the disproportionation of superoxide (i.e., superoxide dismutase activity), participate in the disproportionation of hydrogen peroxide (i.e., catalase activity) and/or halogenate certain organic substrates in the presence of H2O2 (i.e., haloperoxidase activity). We have found that several V(V) complexes catalyze the decomposition of hydrogen peroxide, including V2-transferrin, V(V)-(8- hydroxyuinoline)(O)(OCH3) and V(V)-desferoxamine. The later two are catalyzed by ultraviolet light. We also propose to further characterize the vanadium bromoperoxidase (V-BrPO) isolated from Ascophyllum nodosum and Laminaria setchellii. We have discovered that this enzyme brominates the pyrimidine-containing nucleic acid bases, cytosine and uracil. Experiments are proposed to probe the substrate specificity by variation of the pyrimidine-ring substituents. Experiments are also proposed to probe the function of the vanadium. Does the vanadium act as a Lewis acid catalyst or an electron transfer catalyst, cycling between oxidation states? These mechanistic studies will be further probed using the model complex, V(V)-desferoxamine, which we have shown catalyzes the same bromination reactions as the V-BrPO.